1 Answer

When you see the word "mod" it means you are working as if on a clock. MOD(30) means you are working on a 30 hour clock.

So 67-7 = 60, and on a 30 hour clock every time you go to 30 you arrive at zero. Just as, on a our common 12 hour clock, every 12 hours both hands stand straight up and point to 12, which is the same as zero.